In recent years, the electrophotography has been applied to copying machines as well as various printers since they can give images with high qualities without delay. As a photoreceptor which plays an important role in the electrophotography, the photoreceptor comprising an inorganic photoconductive material such as selenium, arsenic-selenium alloy, cadmium sulfide, zinc oxide and the like has been used. More recently, the photoreceptor comprising an organic photoconductive material was proposed. The latter has the advantages which is not a pollutant and which has a film-formability and a shapability.
As one of the organic photoreceptors, the so-called "laminated-type photoreceptor" in which a charge generation layer and a charge transport layer are successively laminated was developed. The laminated-type photoreceptor is increasingly interested in and is expected to be widely used in the near future because it has the following advantages:
(1) the photoreceptor having high sensitivity can be obtained by suitably selecting and combining the charge generation material and the charge transport material; PA1 (2) the photoreceptor having high safety can be obtained because the charge generation material and the charge transport material can be selected from a wide range of the materials; and PA1 (3) the photoreceptor can be prepared by simple coating and thus it can be prepared with low costs.
However, the prior laminated-type photoreceptors have poor durability. When they are repeatedly used, the electric problems such as the lowering of the charged potential, the accumulation of the residual potential and the change in the sensitivity are caused. The problem as to the accumulation of the residual potential is especially serious because if the residual potential is accumulated, much copies could not be obtained. Such an accumulation of the residual potential is considered to arise from some causes, among which impurities present in the charge transport layer are important. The impurities include impurities originally present in a composition used for forming the charge transport layer, impurities produced after the charge transport layer is subjected to a corona discharge and impurities produced by the decomposition after the charge transport layer is exposed repeatedly during an exposure step and an erasing step and after the charge transport layer is subjected to an outside light during a maintenance operation. These impurities trap carriers so as to produce unmovable space charges which remain as the residual charges in the charge transport layer.
As the other cause of lowering the durability of the laminated-type photoreceptor, the reduction in thickness of the charge transport layer due to mechanical stresses, for example an abrasion such as blade cleaning to lead the lowering of electric properties is mentioned. The increase of the thickness of the charge transport layer is effective for preventing the reduction in thickness of the charge transport layer and increasing the sensitivity of the photoreceptor, but it is accompanied with the increase of the amounts of impurities so that the accumulation of the residual potential makes more remarkable.
For preventing the accumulation of the residual potential caused by the impurities present in the charge transport layer, an addition of a specific compound in the charge transport layer is attempted. However, the prior known compounds are not satisfactory because they prevent the accumulation of the residual potential insufficiently and they affect the electric properties including the charge-ability and sensitivity.
The present inventors have been investigated the specific compound which can prevent the accumulation of the residual potential sufficiently without affecting the electric properties and now they found that metal complexes or salts of a carboxylic acid in which the group "--COOH" directly connects with an aromatic ring satisfy the above requirements.